Light emitting diodes (LED) are an area of interest in the lighting industry due to energy savings among other desirable attributes. More and more legislation is demanding implementation of such systems to replace typical tungsten filament (incandescent) or neon based light structures.
The technology for LED based lighting systems is new and, as such, has constraints which need to be accommodated. For example, conventional incandescent bulbs are designed to accommodate a tungsten filament brought to over 2000° C. through resistive heating inside a vacuum chamber. As such, temperatures on the surface of the bulb can reach many hundreds degrees Celsius, for which black body radiation is an important source of cooling in addition to convection cooling. Over the years such lighting systems have been designed to accommodate these higher temperatures.
An LED lighting system, while generating less waste heat, is much more sensitive to temperatures than those found in incandescent bulbs just explained. And those designing LED lighting systems should strive to efficiently remove whatever waste heat is generated.
An LED light system is typically based on a 3-5 semiconductor doping structure. The ‘three’ designates elements with 3 electrons in an outer valance p shell and five elements are those having 5 electrons in the outer shell. Both elements are most stable chemically with 4 electrons in the shell. When 3 groups and 5 groups are put into close proximity to one another within a substrate, a diode junction is formed as electrons diffuse to fill shells in the 3 group generating an electric field. As an external voltage is applied, electrical current is passed across the junction and under the proper conditions some of the electrical energy is converted to light energy. A fundamental constraint of such systems is that a thermal leakage current component is introduced as temperatures increase. Such currents can disrupt the control of the current voltage relationship used in the control of the LED's light output. Commercial semiconductor devices, for example, are designed to operate with the diode junction temperature well below where black body radiation is significant. Therefore, it is important that both convective and conductive heat transfer principles be used to eliminate waste heat.
The present solution comprises a system of providing thermal backplanes for conduction of waste heat away from an LED or a cluster of LEDs, and toward a manifold employing a passive convective heat transfer system. The manifold comprises multiple chambers being formed by fins projecting inward from an outer cincture or perimeter skirt located about the radial perimeter of the fixture. The perimeter skirt, in addition to creating improved aesthetics by hiding the heat transfer fins, also provides constriction for the airflow and an additional heat transfer surface.
Heat generated through operation warms the surrounding air causing it to rise. This is generally referred to as free convection of a fluid. Free convection can be defined as a passive transfer of heat into a fluid (generally the air) causing differences in density of air that thereby causes the flow of air generally in an upward direction or draft. Cooler air from below rises due to the pressure differential and, in one aspect of the invention, is channeled by a light cover toward a manifold where it is concentrated into a laminar flow directed toward the manifold.
The manifold, comprising a multiple of fins projecting inwardly from the perimeter skirt, constricts the flow at the inlet which then opens up shortly thereafter and by means explained by the Bernoulli's Principle increases the velocity of air across the fins. Under a special set of conditions, the Bernoulli's Principle is manifest as what is known as the Venturi effect.
The fins, in addition to the mechanism explained above, receive heat by thermal conduction from a backplane. In one aspect of the invention, the constriction is followed by an opening or deconstruction. The increased velocity due to the Venturi effect followed by an expansion just beyond the constriction which transitions the flow from laminar to turbulent flow which further enhances the thermal flux to maximize the removal of heat from the fins. Such concentrated and accelerated flows can be referred to here as induced convection heat transfer. To induce generally means to “move by persuasion or influence; to call forth or to bring about by influence or stimulation”. Therefore induced convection can be viewed as “Heat convection in which fluid motion is persuaded or enhanced or influenced by some external agency beyond that provided by free convention”. For present purposes, induced convection can be seen as similar to a forced convection, but without need for motorized or other such mechanical means for stimulating enhanced fluid motion.
In one aspect of this invention a flow with a velocity of between 1 to 2 feet per second can be induced in the region of interest across the fins. This higher velocity flow creates an increased heat flux from the perimeter skirt and the outer perimeter of the fins. In one aspect of the invention heat flux of between 200 to 300 Watts per square meter can be generated. Cooling across the fins caused by the high heat flux creates a high temperature gradient across the fins. In one aspect of the invention, a temperature gradient between 6-7° C. can be generated across each manifold fin, with the lowest temperature being in the perimeter region. Having such a high temperature gradient causes heat to be drawn into the region of high velocity flow and high heat flux.
Those skilled in the art will recognize that the foregoing explanation is for illustrative purposes regarding one aspect of the invention and is not limiting in any way upon the principles taught herein. Further, in this scheme it is anticipated that the higher the temperatures the more active the induced convective cooling becomes.
It is therefore an object of the invention to provide a passive heat transfer thermal management system for a light fixture.
It is therefore an object of the invention to provide a heat transfer system taking advantage of the convective updraft generated by waste heat from the light fixture.
It is another object of the invention to provide a heat transfer system taking advantage of both conductive and convective heat transfer.
It is another object of the invention to provide a heat transfer manifold to aid in convective heat transfer.
It is another object of the invention that this manifold structure provides a thermal perimeter skirt for aiding in heat transfer.
It is another object of the invention that this manifold structure provides multiple chambers comprising vertical fins to aid in heat transfer.
It is another object of the invention that this manifold structure be designed to utilize a venturi effect flow to facilitate cooling.
It is another object of the invention to provide a cooling system for inducing convective heat transfer without mechanical means.
It is another objective of the invention to provide a pleasingly aesthetic cooling system for a light fixture.
It is another objective of the invention to provide a cooling system for a light fixture which is low maintenance.
It is another objective of the invention that the cooling system will work with luminaires that can illuminate large open spaces and provide adequate illumination to those spaces.